CopyOnWriteArraySet

public class CopyOnWriteArraySet extends AbstractSet<E>
implements Serializable

A Set that uses an internal CopyOnWriteArrayList for all of its operations. Thus, it shares the same basic properties:

  • It is best suited for applications in which set sizes generally stay small, read-only operations vastly outnumber mutative operations, and you need to prevent interference among threads during traversal.
  • It is thread-safe.
  • Mutative operations (add, set, remove, etc.) are expensive since they usually entail copying the entire underlying array.
  • Iterators do not support the mutative remove operation.
  • Traversal via iterators is fast and cannot encounter interference from other threads. Iterators rely on unchanging snapshots of the array at the time the iterators were constructed.

Sample Usage. The following code sketch uses a copy-on-write set to maintain a set of Handler objects that perform some action upon state updates.

 class Handler { void handle(); ... }

 class X {
   private final CopyOnWriteArraySet<Handler> handlers
     = new CopyOnWriteArraySet<>();
   public void addHandler(Handler h) { handlers.add(h); }

   private long internalState;
   private synchronized void changeState() { internalState = ...; }

   public void update() {
     changeState();
     for (Handler handler : handlers)
       handler.handle();
   }
 }

Public Constructor Summary

CopyOnWriteArraySet()
Creates an empty set.
CopyOnWriteArraySet(Collection<? extends E> c)
Creates a set containing all of the elements of the specified collection.

Public Method Summary

boolean
add(E e)
Adds the specified element to this set if it is not already present.
boolean
addAll(Collection<? extends E> c)
Adds all of the elements in the specified collection to this set if they're not already present.
void
clear()
Removes all of the elements from this set.
boolean
contains(Object o)
Returns true if this set contains the specified element.
boolean
containsAll(Collection<?> c)
Returns true if this set contains all of the elements of the specified collection.
boolean
equals(Object o)
Compares the specified object with this set for equality.
void
forEach(Consumer<? super E> action)
boolean
isEmpty()
Returns true if this set contains no elements.
Iterator<E>
iterator()
Returns an iterator over the elements contained in this set in the order in which these elements were added.
boolean
remove(Object o)
Removes the specified element from this set if it is present.
boolean
removeAll(Collection<?> c)
Removes from this set all of its elements that are contained in the specified collection.
boolean
removeIf(Predicate<? super E> filter)
boolean
retainAll(Collection<?> c)
Retains only the elements in this set that are contained in the specified collection.
int
size()
Returns the number of elements in this set.
Spliterator<E>
spliterator()
Returns a Spliterator over the elements in this set in the order in which these elements were added.
<T> T[]
toArray(T[] a)
Returns an array containing all of the elements in this set; the runtime type of the returned array is that of the specified array.
Object[]
toArray()
Returns an array containing all of the elements in this set.

Inherited Method Summary